An electrode of a catheter can be formed by an electrically conductive material at a distal portion of the catheter. The electrode material typically is stainless steel or platinum, although other electrically conductive materials can be used. In general, the electrode is a band, or series of longitudinally-spaced bands, at the distal portion of the catheter. An electrical wire typically runs through the catheter and is coupled to the electrode to provide electrical energy to the electrode (ablation) and/or to carry recorded signals from the electrode (mapping). Typical electrodes are between about 2.0 and 10.0 mm in length and between about 0.5 to 3.0 mm in diameter.
Medical mapping and ablation procedures can be performed with such catheters by passing the catheter through a body lumen (e.g., a vein, an artery) to a site of interest (e.g., the interior of a heart). Once the catheter is inserted into the body and the electrode is located at the site of interest, electrical energy can be received or emitted by the electrode to map or ablate, respectively. Mapping generally refers to receiving, with the electrode, electrical signals generated by the body (e.g., endocardial signals generated by the heart) and analyzing those signals to determine the source of a medical problem (e.g., an arrhythmia). Once the source and location of the problem has been identified by mapping, tissue at the location is destroyed by ablation to eliminate the problem (e.g., stop the arrhythmia). To ablate tissue, the electrode is placed in contact with the tissue and energized. The energy source can provide a variety of frequencies of energy including radio-frequencies (RF). Energy sources which provide high voltage direct current shock also have been used to energize the electrode. RF energy sources are preferred in some applications because of advantages (e.g., controllability) over DC energy sources.
With RF energy, however, the lesion which can be created is limited to a relatively small size and depth (i.e., volume). To increase the volume and surface area of the lesion, (i) RF energy can be applied multiple times, or for increased periods of time, to the same location, (ii) the output of the source driving the electrode can be increased, or (iii) the length and/or diameter of the electrode can be increased. While each of these three approaches have been attempted, none is entirely ideal. With multiple or extended applications of RF energy, time is lost performing the same operation two or more times or for extended periods of time, and accuracy can suffer because of unintended movement of the electrode between applications. When the output of the source driving the electrode is increased, lesion enlargement occurs but only up to a point because desiccation of tissue causes an abrupt increase in the impedance at the electrode/tissue interface which limits energy transfer to the tissue. Increasing the length and/or diameter of the electrode beyond a certain point (e.g., beyond about 4.0 mm in length and about 2.0 to 3.0 mm in diameter) is not a satisfactory solution because with an increase in surface area of the electrode comes a corresponding decrease in current or power density delivered to the electrode/tissue interface (assuming the output of the source driving the electrode remains substantially constant or limited) and a greater proportion of the electrode not in contact with the location to be ablated.
While standard electrode configurations (e.g., a band, or series of longitudinally-spaced bands, located at a straight, distal portion of the catheter) allow recording along only the longitudinal axis of the straight electrode, they cannot record signals in other dimensions. Thus, in general, electrodes having standard, straight configurations cannot record over large surface areas or volumes of tissue. Similarly, performing ablation with such standard electrodes results in lesions along only the straight longitudinal axis. Lesions having large surface areas or volumes cannot be created because the standard, straight electrodes can neither cover other dimensions nor concentrate energy at the electrode/tissue interface.